1. Field of Invention
The present invention relates to switch power supply, more particularly, relates to a switch power supply having current overloading proof function and it's IC.
2. Description of Related Arts
Switching power converters are used in a wide variety of applications to convert electrical power from one form to another form. For example, DC/DC converters are used to convert DC power provided at one voltage level to DC power at another voltage level and AC-DC converters are employed to convert alternate current power into direct current power. At the same time, switching power converter could be categorized into isolated or non-isolated power converter, and the basic circuit of the converter can be configured to step up (boost), step down (buck), or invert type, even CCM (continuous conduction mode) or DCM (discontinuous conduction mode).
The isolated power converter could be further classified into single ended mode (including forward and flyback converter) and double ended mode (push-pull, half bridge and full bridge converter); the converting technique comprises hard-switched converters and soft-switched converters, and the controlling techniques comprise PFM (Pulse Frequency Modulation) mode control, PWM (Pulse Width Modulation), current mode control, voltage mode control and so on.
Regardless what methods or mode are used, a switching power circuit generally comprises a converter circuit having one or more field effect transistor, a transformer or an inductance, and at least one rectifying filter output circuit, wherein the quantity of the field effect transistor is subject to the choice of power converter mode, commonly, single ended converter comprises a field effect transistor, the double ended converter comprises a plurality of field effect transistors. In case of the soft switch is applied, at least one more supplemental field effect transistor is necessary. The inductance here is being used for the simple non-isolated DC/DC converter, while the choice of the chosen converter will simultaneously determine whether the inductance, single-ended or double-ended mode, hard switching or soft switching, to be applied in practice.
Further, the switching power circuit comprises a feedback circuit having a sample circuit, an error amplifier, and occasionally a feedback isolating circuit, wherein the sample circuit is adapted for sampling the current and voltage signal from the output circuit, and sending the sampled current and voltage signal to the error amplifier to obtain a comparative value, afterwards, the error amplifier will output an error signal.
Additionally, the switching power circuit comprises a control circuit including an adjustable pulse circuit and a drive circuit, wherein the adjustable pulse circuit having PFM (pulse frequency modulation) mode, PWM mode and so on. According to the error signal, the adjustable pulse circuit is capable generating a basic pulse, for double-ended mode, there is a scaling-down complementary double pulse circuit, for soft switching multi-pulse circuit, there is a multi pulse circuit. Commonly, basic pulse, double pulse and multi-pulse are supposed to be directed into the driven circuit. It is noted that a bigger error signal will result to a larger duty cycle ratio, as well as a higher peak value of the field effect transistor current and a saturation susceptible transformer.
Finally, the switching power circuit also comprises an supplemental circuit which is selected from a group consisting of initiating circuit, protective circuit, voltage reference circuit, EMC circuit, and alternate rectifying filter circuit, wherein the protective circuit could be further classified into the lower voltage protective circuit, high voltage protective circuit and upper limit current protective circuit. Whenever the switch power supply is initiated or overloaded, the transformer and induction is susceptible to be saturated, and field effect transistor is apt to be loaded with over current. So within the art, the power switching IC employs the upper limit protective circuit for protection, that is to say, when the current reach the upper limit, the field effect transistor will be automatically shut off. Therefore, it is required that the control circuit to be promptly responsible and the field effect transistor be equipped with instantaneously shutting-off function. Otherwise, there exist some sort of hidden risks for the field effect transistor and transformer. For the initiating circuit, there are resistance initiating circuit and switch-off constant current source initiating circuit available within the art.